1. Field of Invention
This invention relates to thermal energy storage systems and is particularly directed to a liquid-solid thermal energy storage system having a thermocline.
2. Description of the Prior Art
As the world's population becomes aware of the decline of the world's oil reserves, there is developing increasing interest in alternative energy systems. Thus, considerable study is being directed to solar energy systems. However, due to dirunal temperature changes, cloudiness and various other matters, the problem of thermal storage has been one of the major problems. In this connection, the late Dr. Farrington Daniels, noted physical chemist and past president of the Solar Energy Society, presents an excellent morphological survey of thermal energy storage concepts in his book, "Direct Use of the Sun's Energy, " Yale University, 1964. Dr. Daniels divides thermal energy storage concepts into three basic categories:
1. Sensible Heat -- Storage by heat capacity
2. Physcial Changes -- Particularly heats of fission and/or vaporization
3. Reversible Chemical Reaction
When economic analysis is made of these three categories, it becomes apparent that the two major considerations are: the inventory cost of the heat storage medium and container and, secondly, the engineering complexity of inputting and extracting the heat. Most analyses tend to favor sensible heat storage as the most economically and operationally attractive system. For temperatures up to about 200.degree. F., water is by far the best medium. To quote Dr. Daniels, "Water has about the highest heat capacity per kilogram per liter or per dollar of any ordinary material." The next lowest price heat storage medium is gravel or crushed rock, which is available at a cost of a few dollars per ton and is suitable for storage of heat at temperatures up to at least 1500.degree. F., the upper limit being determined by the fluid which flows through the rock bed to input or extract heat. Rock has been used as a heat storage medium for many years in "pebble bed" heaters, in which a gas (usually air) flows through the bed to input or extract heat. A major limitation of such heaters is that they do not have a thermocline, and the temperature of the exiting hot gas during heat extraction decreases rapidly from the storage temperature. It is known that water (and other liquids) may produce a "thermocline", that is, the hot and cold water may be made to separate into layers having a fairly distinct boundary, which rises or lowers within the container as water is added or withdrawn, so that the temperature of the water being drawn off can be substantially constant until the thermocline is reached and, at that point, will drop sharply to the temperature of the unheated water. This phenomenon is familiar in domestic hot water heaters. In contrast, where rock is the thermal storage medium in "pebble bed heaters", the temperature tends to increase gradually when heat is input, and to decrease gradually, as heat is withdrawn.
It will be apparent that the thermocline principle is advantageous, but has, heretofore, been limited to relatively low temperature, all liquid thermal storage systems, whereas rock has been capable of storing heat at considerably higher temperatures but has been subject to supply degredation in temperature.